Detroit – So engineers think 35 mpg by 2020 is difficult? The EPA’s Margo Oge created a stir at this week’s Society of Auto Engineer’s convention when she said the nation’s passenger vehicle fleet may have to average 75 mpg by the 2030s. That figure, Oge explained, would be necessary to satisfying Green demands that greenhouse emissions be cut 80 percent by 2050.

So the question was posed to the SAE panel of engineers I attended: Is there any technology on the horizon that will get us there?

The answer was no.

Pols and their pipe dreams aside, spending time with auto engineers brings home the reality of why the internal combustion engine (and its diesel cousin) have dominated transportation for over a century. The ICE is cheap, compact, and its fuel source is infrastructure-friendly.

The panel then explained why the major alternatives are just that – alternatives. (Interestingly, they didn’t even find ethanol worthy of discussion.)

Lithium ion batteries? (For use in plug-in vehicles like the Chevy Volt due in 2010)

“It is not the breakthrough you need,” explained Andreas Schamel, Ford’s Chief Engineer for Engine Engineering. “On efficiency, range, and compactness, it is not even close to the current hydrocarbons.”

Toyota engineer Tom Stricker: “Batteries will have to last the life of the vehicle.” Something consumers assume for ICE engines – but that will take years for plug-ins to prove out.

Added Chrysler’s Robert Lee: “Consumers think that electricity is free right now. They don’t know where it comes from.” He said the U.S. would need a large expansion in coal-powered plants to feed the mass sale of plug-ins. And the same green lobby that likes plug-ins despises coal.

How about hydrogen? (Routinely referred to by the press as the fuel of the future)

Andreas noted that “You can already use hydrogen in existing powertrains” (BMW has done a lot of work with its 7-Series, for example). The trouble is, where does the fuel come from?

“If you generate it from coal, then you haven’t gained anything,” says Schamel, referring to CO2 emissions. His point is known to engineers but little understood by the public: Separating hydrogen from water – so-called electrolysis – is a very electricity-intensive process.

Continues Schamel: “The key to hydrogen is developing a nuclear power infrastructure that will separate the H from the H2O.”

Anybody heard a green congressman touting his plan to build dozens of nukes by 2030?